Cannot save or open files with Flatpak app

[Simone R.]

As stated on the Flatpak's app issues page ( https://gitlab.gnome.org/World/apostrophe/-/issues/415 ) I cannot either load or save files.

ProblemType: Bug
DistroRelease: Ubuntu 21.10
Package: python3.9-minimal 3.9.7-2build1
ProcVersionSignature: Ubuntu 5.13.0-52.59-generic 5.13.19
Uname: Linux 5.13.0-52-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu71.2
Architecture: amd64
CurrentDesktop: KDE
Date: Fri Jun 24 21:29:55 2022
ExecutablePath: /usr/bin/python3.9
InstallationDate: Installed on 2022-06-06 (18 days ago)
InstallationMedia: Kubuntu 21.10 "Impish Indri" - Release amd64 (20211012)
LocalLibraries: /app/lib/libgspell-1.so.2.3.0 /app/lib/libhandy-1.so.0
Python3Details: /usr/bin/python3.9, Python 3.9.7, python3-minimal, 3.9.4-1build1
PythonDetails: N/A
SourcePackage: python3.9
UpgradeStatus: No upgrade log present (probably fresh install)